Polymeric sheetings have been used in situations where a frangible signage article is desired. Frangible signage articles often are capable of being affixed to a surface, yet are easily damaged or torn when an attempt is made to remove or alter them. Frangibility is generally desired for security reasons, for example, to inhibit fraud, and may be provided to a signage article by thinness, deformability, strong adhesion to the substrate to which an article is affixed, brittleness, and/or ease of separation of various layers of an article.
Polymeric sheetings also have been used to produce signage articles that have retroreflective capabilities. An article possesses a retroreflective capability when it can return a substantial portion of incident light in the direction from which the light originated. Retroreflectivity renders enhanced conspicuity to the article in low or restricted lighting situations, or in situations where sheeting materials must be viewed from a distance.
Polymeric sheetings also have been used to produce signage articles that have good durability. Durability of a signage article may be important in situations where the article may be exposed to harsh vapors, ultraviolet light, temperature or humidity extremes and the like. Abrasion resistance and resistance to cleaning agents and the solvents used in cleaning solutions also are, in some cases, important aspects of durability. If extended useful life is not an important consideration (such as labels for rapid turnover packaging), lower cost non-extended life sheetings may be used.
Polymeric sheetings also have been used to form signage articles having indicia such as alphanumeric characters, bar codes or graphics. Frequently, the signage articles will carry information that is repeated or incrementally varied over a large number of items; for instance, license plate validation stickers may have state or county identifying information repeated on a large number of validation stickers.
Indicia may be printed on the polymeric signage articles by such well known processes as letter press, offset press, screen printing, or hot foil stamping. These types of printing processes normally provide satisfactory print quality, legibility, and adhesion; however, the equipment for these processes can be relatively expensive. In addition, when using letter press and offset press printing, print plates or rubber blankets must be prepared, and when using screen or hot stamp printing, a screen or hot stamp, respectively, must be prepared. The preparation of the plates, blankets, screens, or stamps can be a costly, time-consuming process. In many cases, a solvent-borne colorant is used, which requires disposing of the solvent in an environmentally-sound manner. Known processes also may necessitate the use of drying ovens and may require a certain amount of drying time. Further, the known means of printing indicia on articles are limited by the ease (or lack thereof) with which the information on individual items can be varied.
The documents discussed below contain disclosures that may be pertinent to the present invention.
U.S. Pat. No. 5,118,930 to Takada discloses a retroreflective sheeting using a polyvinyl chloride topmost layer upon which bar codes may be directly thermally printed.
Commonly-assigned Patent Cooperation Treaty application to Goeb, having international publication number WO 93/12155, discloses label stock having a halogen-free acrylic urethane topmost layer. The application discloses that the additional step of priming the upper surface of the label stock, for example, by corona discharge or with a diluted solution of an acrylate polymer or adhesive, is preferred in order to promote adhesion of inks.
Compositions containing aqueous dispersions of polyurethane and a crosslinker are known to be useful as prime layers between the component layers of multilayer cover films of retroreflective sheetings, or as a prime layer to improve the adhesion of a cover layer to underlying components of the retroreflective sheeting. For instance, commonly-assigned U.S. Pat. No. 4,896,943 to Tolliver et at., discloses encapsulated-lens retroreflective sheetings and the use of a composition containing urethane and crosslinker as a prime layer about 2 microns thick, to improve the adhesion of a cover layer to an underlying binder layer. In addition, commonly-assigned U.S. Pat. No. 5,066,098 to Kult et al., discloses the use of a similar composition as the inner layer of a cover film of an encapsulated-lens retroreflective sheeting.
The above-cited Tolliver patent also discloses the subsequent application of inks to the upper surface of an encapsulated-lens retroreflective sheeting. The Tolliver patent suggests the use of a urethane and aziridine composition to form a conditioning layer on the outside surface of a dual layer cover film of a retroreflective sheeting. However, direct thermal printing is generally difficult on such encapsulated-lens sheetings because of their undulating upper surface. It is known to use a prime layer of a polyurethane about 2 microns thick over an enclosed-lens retroreflective sheeting that has a cover layer such as ethylene/acrylic acid.
U.S. patent application Ser. No. 08/030,672, filed Mar. 12, 1993 by Watkins, et at., (commonly assigned and incorporated herein by reference) discloses compositions comprising an aqueous aliphatic or aromatic polyurethane dispersion and an acrylic emulsion for forming clear coats at least ten microns thick over finished frangible articles having indicia printed thereon. Such clear coats are desired in order to bury or embed the indicia. Watkins, et at., also discusses roll coating of indicia onto embossed, nonfrangible articles having such clear coats.
Commonly-assigned U.S. Pat. No. 4,767,659 to Bailey et at. (incorporated herein by reference) discloses enclosed-lens retroreflective sheetings having a thermoplastic cover film comprised either of aliphatic urethanes, copolymers of ethylene or propylene, or homopolymers of ethylene and propylene. The thermoplastic cover films are formed by extruding onto a carrier web and laminating to a retroreflective base material. The thermoplastic cover films are sufficiently extensible to withstand substantial stretching such as may occur during the embossing of a license plate. The materials also may serve to protect the underlying material against weathering.
FIG. 1 shows a known retroreflective sheeting 12 comprising removable protective liner 14 at the bottommost side, retroreflective element 16, pressure sensitive adhesive layer 26, polyethyleneterephtalate (PET) layer 18 with a thickness of about 25 microns and colorant/binder receptive print layer 20. Retroreflective element 16 comprises a monolayer of glass microspheres 30 embedded in a layer of polyvinyl butyral 34 with underlying reflective layer 32 and pressure sensitive adhesive layer 36. Layer 20 is directly thermally print receptive with a resin-based indicia and is formed from a composition comprising PET and a vinylidine/acrylonitrile copolymer. Sheeting material 12 (manufactured for use as an indoor product ScotchMark.TM. brand label stock 3929 by Minnesota Mining and Manufacturing Company, St. Paul, Minn.,) is not sufficiently frangible or durable, as defined below, for many applications.